Code division multiple access (CDMA) communication system is adopted in second-generation mobile communication systems such as IS-95, in third-generation mobile communication systems such as IMT-2000, and in wireless LANs. Generally, in conventional CDMA communication system, binary phase-shift keying (BPSK) or quaternary (quadrature) phase-shift keying (QPSK) is utilized as primary modulation.
FIG. 6 is a block diagram illustrating the structure of modulator/demodulator of CDMA communication system utilized in third-generation mobile communications as a conventional technique. The modulator/demodulator is one well known to those skilled in the art, e.g., in case of IMT-2000 illustrated in “Simple IMT-2000” by Kota Kinoshita (Chapter 5, published by the Electrical Communications Society, 2001). QPSK(Quaternary Phase-Shift Keying) is used as a primary modulation in a downlink and BPSK(Binary Phase-Shift Keying) as primary modulation in an uplink.
On a transmit side, primary modulation of a carrier signal by transmit information is carried out by a modulator 1A that performs BPSK or QPSK, the BPSK- or QPSK-modulated signal is spread-spectrum modulated (subjected to secondary modulation) with a spreading code by a spreader 2A, which outputs the spread-spectrum modulated signal as a transmit signal. On a receive side, the spread-spectrum modulated signal is supplied as a received signal to a despreader 3A which despreads the received signal with a spreading code to output the BPSK- or QPSK-modulated signal, and the BPSK- or QPSK-modulated signal is demodulated by a demodulator 4A, which outputs the demodulated signal as received information.
FIG. 7 is a system block diagram illustrating the principles of IMT-2000 DS-CDMA [“Simple IMT-2000” by Kota Kinoshita (published by the Electrical Communications Society, 2001)]. In application of error-correcting encoding as channel encoding, use is made of convolutional code and turbo code is applied in case of high-speed data. A narrow-band modulator 1B employs QPSK in the downlink and BPSK in the uplink. FIG. 7 illustrates the uplink from a user terminal. At the user terminal, transmit data is modulated by a BPSK modulator 1B, the spectrum of the modulated signal is spread with a specific spreading code by a spreader 2B, the spread-spectrum signal is converted to radio frequency by a frequency converting circuit 7B, and the resultant signal is transmitted from an antenna as a wide-band radio signal. On the receive side, a wide-band radio signal is received from each user terminal, the received signal is despread with a spreading code of each user terminal via a frequency converting circuit 8B and filter, each signal is demodulated by a narrow-band demodulator 4B, and the resultant signal is output as received data.